Originally Posted by
corporate-wage-slave
Longhaul can be from A, B or C, and you won't know until 80 minutes or so before departure, unless you get a tip off from lounge or check in staff before then. The A380s are almost certain to leave from C, but I'm sure there will be boarding from A or B at some point, even if it is a one off. So all you can plan to be in South Lounges Complex, and all parts of T5 are within 15 minutes walk from there anyway.
As can all short haul!! I have also boarded/disembarked A380's from B. This morning I came in to one of the regular C A380 gates on BA26.